oil spill. Much of the data for establishing baseline conditions—the status of the ecosystem had the spill not occurred—need to be collected as soon as possible after the start of the spill and before the effects are manifested. Without this effort, most regions would have insufficient observations to document the conditions of habitats and species at the time of the spill; an important consideration given that ecosystems are not static and change in response to other human activities. Often the closest that one can come to evaluating conditions post facto is to assemble evidence on conditions prior to the spill, or “baseline conditions.” Chapter 3 discusses baseline conditions in the Gulf of Mexico (GoM). As outlined in Chapter 1, an issue with using baseline conditions to assess what the present situation would be without the oil spill is that there are numerous other dynamic processes besides the oil spill that affect ecosystems and the services they provide, complicating the task of isolating the impact of the oil spill.

To measure the impact of human actions, either intentional actions brought about by a policy or management change, or unintentional actions, like an oil spill, requires understanding three important links (Figure 2.1). First, what are the impacts of human actions on environmental conditions that affect the structure or function of ecosystems? Ecosystem functions describe the internal processes of the ecosystem (e.g., energy fluxes, nutrient recycling, food-web interactions) while structure refers to the organization of the biophysical components that determine ecosystem functioning. Much of the current scientific work in the Gulf is an attempt to assess the chemical, physical, and biological impacts of the oil spill—an important first step in discerning the impact on ecosystem structure and function. Natural disturbances, such as a hurricane or changes in precipitation that affect discharge from the Mississippi River, also have impacts that lead to changes in environmental conditions. Again, as discussed in Chapter 1, natural variation can make it difficult to disentangle impacts of the oil spill from other factors influencing environmental conditions. This reality will be a fundamental challenge in all work related to the GoM oil spill.

Second, how do changes in the structure and function of ecosystems lead to changes in the provision of ecosystem services? “Ecological production functions” are used to describe the provision of ecosystem services as determined by the structure and function of ecosystems. They can be thought of as a “transfer function” or model that quantitatively describes the inter-relations of the ecosystem in such a way that changes in ecosystem condition (e.g., loss of habitat) translate to impact on ecosystem services (e.g., shrimp fishery yields). Ecological production functions can be used to predict how the provision of various ecosystem services changes with

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